Field of the Invention
The present invention relates to a method for controlling a valve lift device. A device such as this is employed in particular in internal combustion engines fitted to motor vehicles.
Valve lift devices whose function is to correctly manage the opening and closing of the inlet and exhaust valves mounted on the cylinders of a combustion engine are already known. To do this, these valve lift devices generate a predetermined timing angle between the camshaft and the crankshaft.
Until now, for each type of engine, the set point timing angle to be applied was determined on a test bed as a function of the engine speed, the amount of air let in, and the position of the throttle valve. This set point value was applied via the valve lift device. The actual timing angle between the camshaft and the crankshaft was constantly checked and if the actual timing angle and the set point timing angle did not coincide, a correction was applied.
It soon became apparent that with this method of operation, it was necessary to keep on correcting the relative timing angle of the camshaft and crankshaft. Furthermore, the accuracy and stability of a timing angle set as a set point, and the speed with which this set point value is reached was not satisfactory.
The purpose of the present invention is to improve the control of the valve lift device and, in particular, to improve its precision, stability and speed.
To this end, the present invention relates to a method for controlling a valve lift device associated with an internal combustion engine, this method being of the type consisting in determining, according to appropriate maps, the relative timing angle to be applied to a camshaft and a crankshaft and in applying this timing angle via a control of the PID type, said method being characterized in that it consists in:
(a) determining the reference value xcex10 of the timing angle when the valve lift device is inactivated, each time the engine enters stabilized low idle speed, and taking this into consideration when any later timing angle command is given, and in that it comprises at least one of the following steps:
(b) pre-establishing, on the test bed, a map of a duty cycle (RCO) for the opening of a hydraulic valve controlled by the PID of the valve lift device as a function of the engine water temperature, and adjusting the timing angle to be applied (by modifying the reference value xcex10) as a function of this water temperature,
(c) determining the difference between the command (RCO) applied and the command calculated at step (b) by observing the I term of the PID controlling the valve device and storing its value in an adaptive term,
(d) determining the response time of the valve lift device following a determined command step and as a function of this response time, modifying the proportional term P of the PID,
(e) checking the relative timing angle of the camshaft and the crankshaft does not fluctuate when the set point value is stable, and accordingly modifying the P term of the PID to avoid this fluctuation.
The method according to the invention thus makes it possible, on the one hand, to improve the calculation of the timing angle a to be applied by the valve lift device, and also to correct the P and adaptive terms of the PID controlling this valve lift device. The fact of measuring the value xcex10 of the angle between the camshaft and the crankshaft, in the absence of any activity of the valve lift device, each time the engine is in a stabilized low idle period, makes it possible to take account of the change in this timing angle throughout a journey, rather than simply upon start-up. This timing angle does actually tend to change with the engine temperature, and capturing it just once at start-up is insufficient.
Furthermore, the method according to the invention also makes it possible to diagnose malfunctionings of the valve lift device in static or dynamic regime. Specifically, the response times of the valve lift device for performing a predetermined command step are measured and analyzed. If these response times are too long, a valve lift device malfunction indicator is activated.
Likewise, a malfunction in static regime is detected when the difference between a set point timing angle and the measured timing angle is greater than a threshold a certain number of times.
Other objects, features and advantages of the present invention will in any case be better understood from reading the description which follows, with reference to the figure of the drawing.